1. Technical Field of the Invention
The present invention relates to main spindle devices for removably attaching tools to the spindles of machine tools.
2. Background Art
As a conventional main spindle device for machine tools, the type shown in FIG. 8 is commonly known. This main spindle device is inserted into a main spindle 31, and is composed of a pipe-shaped draw bar 32 which moves right and left in the view shown in FIG. 8 by means of a motion means, a tube member 33 affixed to the tip of the draw bar 32, and a collet 34 with an expandable diameter fitted onto the outer circumference of the tube member 33. Pressure applying surfaces 33a and 33b which gradually expand in approaching the tip are formed on the outer circumferential surface of the tube member 33, pressure receiving surfaces 34a and 34b which gradually expand in approaching the tip are formed on the inner circumferential surface of the collet 34, and an anchoring portion 34c is formed on the outer circumferential surface of the collet 34.
When attaching a tool to the main spindle device, the tool holder T is first inserted into an attachment hole 31a in the main spindle 31, and a coolant connecting tube portion Ta is fitted onto the tube member 33 (see the top half of FIG. 8). Then, upon moving the draw bar 32 to the right in FIG. 8, the pressure receiving surfaces 34a and 34b are pressed against the pressure applying surfaces 33a and 33b so as to expand the diameter of the collet 34. As a result, the anchoring portion 34c is engaged with the anchoring end Tb of the tool holder T so as to anchor the tool holder T to the main spindle 31 as shown in the bottom half of FIG. 8.
However, the conventional main spindle device described above has a structure wherein the coolant connecting tube portion Ta is fitted onto the tube member 33 when the tool holder T is inserted into the attachment hole 31a of the main spindle 31. For this reason, the tool T must be moved linearly and precisely so as to be aligned with the central axis of the main spindle 31 (tube member 33). Therefore, when using an automatic tool exchange apparatus which inserts tools into the attachment hole 31a of the main spindle 31 along a curved path, the gap between the coolant connecting tube portion Ta and the tube member 33 must be enlarged, thereby making it difficult to seal the coolant.
Additionally, with the above-described conventional main spindle device, the retraction of the draw bar 32 causes the collet 34 to expand and the tool holder T to contact the main spindle 31 simultaneously. Therefore, the maximum amount of retraction of the draw bar 32 is restricted by the amount of retraction of the collet 34 and the final amount of insertion of the coolant connecting tube portion Ta into the tube member 33. Consequently, it becomes difficult to increase the magnifying ability by reducing the angle of inclination between the pressure applying surfaces 33a and 33b, and the pressure receiving surfaces 34a and 34b. Moreover, since the allowance of the clamp between the pressure applying surfaces 33a and 33b and the pressure receiving surfaces 34a and 34b is determined by the widths of the gaps A and B, if the positions of the boldface lines in FIG. 8 are not aligned, the gaps A and B will disappear and clamping will not be possible. Thus, good work precision is a stringent requirement.
Furthermore, a disc spring is used to reinforce the anchoring force of the collet 34 on the tool holder T by increasing the retraction of the draw bar 32, but this makes vibrations more likely to occur during high-speed rotation of the tool because the disc spring 35 is a non-continuous element. Additionally, when the tools are removed from the main spindle 31 by contracting the collet 34, hysteresis causes a large return force to be necessary, and the amount of work increases. Moreover, disc springs 35 have comparatively short lifespans, and their maintenance requires care.
On the other hand, the guiding precision of automatic tool exchanging devices is comparatively low, and the tool insertion path with respect to the main spindle 31 easily becomes misaligned with the central axis of the tube member 33. Consequently, the inner circumferential surface of the tube member 33 and the sealing member 36 attached thereto tend to become worn quickly, not only in the case of the above-described tool exchange devices which trace a curved insertion path, but also in the case of tool exchange devices with linear insertion paths, and these also require care for maintenance.